Electric cables under pressure



Aug. 4, 1964 c. A. FLAMAND 3,143,591

ELECTRIC CABLES UNDER PRESSURE Filed July 25. 1960 S Sheets-Sheet 1 Aug. 4, 1964 C- A. FLAMAND ELECTRIC CABLES UNDER PRESSURE 3 Sheets-Sheet 3 Filed July 25, 1960 Unit d S t Pat n 0.

3,143,591 ELECTRIC CABLES UNDER PRESSURE Charles Andr Flamand, Paris, France, assignor to Trefileries ct Laminoirs du Havre, Paris, France, a corporation Filed July 25, 1960, Ser. No. 44,913 Claims priority, application France Nov. 9, 1959 5 Claims. (Cl. 174--15) This invention relates to electric cables under liquid pressure, end-boxes for said cables and the auxiliary devices related therewith.

It is known that the high-voltage behaviour of electric cables in which the insulator consists of a wound tape is promoted by the application of a continuous pressure within the casing accommodating the cable. This may be achieved by pressurizing the liquid (notably oil) which impregnates the insulating tapes, when all the inter-tape spaces are filled with said liquid.

Another solution applicable to cables with wound insulating tapes is to use a gas under pressure, such as nitrogen, which penetrates through the spaces between the tape turns.

However, ditferent conditions appear when the cable insulator, instead of consisting of helically wound tapes, is in the form of a thick sheath of thermo-plastic material formed by extrusion or some similar process. As a mat:

ter of fact, the use of oil under pressure is prohibited in this case, for said substance is liable to have a chemical action on the thermo-plastic sheath.

- On the other hand, experiments have shown that pressure oil (or any similar liquid which is substantially nonvolatile and contains insulating substances) does not pre' vent the gaseous occlusions within the thermo-plastic sheath from being ionized, said ionization being liableto cause rapid destruction of the thermo-plastic sheath.

Under such conditions, said sheath becomes rapidly impaired.

The French Patent No. 1,178,121 to the same assignee, discloses cables with a solid insulating sheath, wherein a pressure applied by an inert gas such as nitrogen is used for increasing the service voltage of the cable, all other things being equal. gas under pressure is efifective to raise the ionization level of the gaseous occlusions within the sheath, in spite of the substantial thickness of the latter.

The present invention is based on the discovery that, by substituting for the previous gaseous fluid a liquid meeting predetermined physical requirements, it is possible to increase the service voltage of the cable for any given pressure value or else, all other factors being equal as compared with a cable having a thermoplastic sheath under gas presure, to increase the life of the cable.

The improvements provided by the present invention still make it possible, at will, either tolower the pressure requirements for the cable or, with the same pressure, to reduce the thickness of the insulating sheath.

Under certain conditions, said improvements more? over allow for a particularly efficient cooling of the cable, whereby the permissible current density may be increased.

According to the invention, the improvements in or relating to electric cables comprising at least one core conductor surrounded by a solid insulating sheath made of thermo-plastic material which is maintained under a fluid pressure, are essentially characterized in that both faces of said thermo-plastic sheath are subjected to a pressure exerted by a vaporizable liquid having low current-conducting properties, the vapor of said liquid being capable of diffusing through the sheath of thermo-plastic material.

In one advantageous embodiment of the invention, the

In fact, it has been determined that 3,143,591 Patented Aug. 4, 1964 pressure liquid contained in the cable consists of pure water, i.e., water whose electrical resistivity is substantially comprised between 0.1 and 2 megohms-cm. cm.

. Experiments have evidenced the surprising fact that the cable behavior under high tension is improved by the pressure liquid in spite of the gaseous occlusions in the sheath, precisely by vapor pressurizing the gaseous vacuoles in said sheath. Moreover, water increases the overall thermal capacity, thus allowing for an increase in the permissible overloads.

Other particular features of the invention, concerning especially the structure of the cable and end-boxes, will appear in the course of the following description.

In the accompanying drawings, given by way of nonrestrictive examples, there is shown a number of different embodiments of the invention.

More precisely:

FIG. 1 is an axial sectional view of a single-pole cable according to the invention.

FIG. 2 is a cross-sectional view of the same cable, taken along the line II-II of FIG. 1.

FIG. 3 is a cross-sectional view of a three-pole cable arranged in accordance with the invention.

FIG. 4 is an elevational view of an end-box for the cables of the above type in axial section and with parts broken away.

FIG. 5 is another elevational view, partly in section, of two end boxes for a high-voltage cable, arranged accordment 1 is a bundle of wires 3, preferably made of copper (or, if desired, of aluminium) which, in the example shown and as an optional feature, are coated with a watercarbon black particlesincorporated within its components.

Resting on the shield 4 is the insulating sheath 5 consisting of polyethylene, either pure or compounded with butyl r The Technology and Uses of Ethylene or polybutylene or with another insulating thermo-plastic material such as polyvinyl chlorides at various stages of polymerisation and their copolymers.

More precisely, the polyethylene above referred to is a solid polymeric material formed by polymerisation of ethylene at either low or high pressure in the presence of catalysts. The molecular weight may be comprised between 12,000 and 30,000 or more, according to internal viscosity measurements.

' Union Carbide Plastics Co., examples of which are DFD 2005, 4100, 6005, 6015, 6505; those sold by Pechiney Company such as Nathene C; those sold by E. Ldu Pont de Nemours and Co. such as Alathon 4 and 5.

The polypropylenes are also convenient for constitut-, ing the cable sheath, examples of which are those sold by the Shell Company and Montecatini Company, such as Moplene AAD.

The polyvinyl chlorides above referred to are' a. polymeric material obtained by polymerisation in the presence of catalysts of the gas resulting from the, reac-.

tion of hydrochloric acid with acetylene.

The polymeric products particularly convenient are those having a K-wert in the range of 70. Such polymers are more fully described in the book The Vinyl Chloride and Its Polymers by Gibello, Dunod, editor, Paris 1959. The sheath 5 may also be made of the various products derived from polyvinyl chloride by substitution and/ or copolymerisation.

Specific examples of commercially available polyvinylchlorides or copolymers are those sold by B. F. Goodrich Company, such as Geon 101 and 404, those sold by the Chemische Fabrik Hochst, such as Hostalit Z, and those sold by Pechiney Company such as Afcovyls C 1200.

All these polymeric materials, which are well known in the art by the cable manufacturers, may be combined in a known manner with various fillers such as carbon black, alumina, calcium silicates, with agents such as plasticizers (specific examples of which are phthalic esters, such as diisooctylphthalate and didecylphthalate), stabilizers, antioxygen, or lubricants (such as those sold by the National Lead Company under the names Dythal and D5 207). These various substances are insoluble in water and remain unaffected even after long immersion in water.

A few examples of insulating compositions for the sheath 5 are given hereunder:

1st Example: Polyethylene Compound Parts by weight Polyethylene or polypropylene of high or low density 100 Polyisobutylene or butyl-rubber to 25 Anti-oxygen (for example,

dibetanaphtylparaphenylenediamine) 0.1 to 1 2nd Example: Non Plasticised Vinyl Polychloride Compound Parts by weight Vinyl Polychloride (or copolymer) of the so-called high impact type 100 Stabiliser (for example lead dibasic phthalate) 2 to 10 Extrusion lubricant (for example,

lead stearate) 0.5 to 2 3rd Example: Plasticised Vinyl Polychloride Compound Add to the above composition of Example 2:

. Parts by weight Phthalic esters (for example) 10 to 50 The sheath 5 is preferably formed by extrusion in one single stratum. It is of substantial thickness, the latter being a function of the voltage provided across the faces of the sheath, numeric examples thereof being given hereinafter. The sheath 5 is coated on its outer face with a second conducting shield 6, made in this case of wound metal taping adapted to allow water filtering. The shield 6 is intended to equalize the potential over the whole cable length.

The thus constructed continuous assembly is housed within a fluid-tight casing 7 adapted to withstand internal pressures of the order of 1 to 25 kg. cm. For singlepole cables (FIG. 2), the casing 7 may be made, for example, of aluminium, hooped lead or of an insulating material (polyethylene, polyvinyl chloride or polyesters, whether or not reinforced with glass fibers). caseof multiple cables, the casing 7 may also be made of steel, as will be set forth hereinafter.

The internal diameter of easing 7 is slightly greater than the external diameter of sheath 5, so that'an an-' nular space 8' of the order of one millimeter is created.

In the purifiers with ion exchangers.

According to the present invention, the duct 11 and the annular space 8 are occupied by a liquid under pressure, these two spaces allowing the transmission of pressure all along the cable. In the example illustrated, the liquid used consists of pure water, either distilled or purified by means of any known apparatus, such as the chemical The purity of the water, as measured by the resistivity thereof, should range from 0.1 to 2 megohms-cm. cm. If required, any agent not liable to be electrolysed, e.g. an antifreeze such as ethylene-glycol may be added to said water.

The water contained in the cable is pressurized by known means arranged on the cable or at the ends thereof. Thus, the pressure may be built up by a set of tanks or pumps, or else by resilient containers or pressure gas cushions. Systems, which may or may not be coincident with the first-mentioned members, are also provided for absorbing the variations in the volume of the water, resulting from the expansion of the latter during the heating cycles. Examples of such systems will be given in FIGS. 4 and 5.

The water pressure value is calculated in economic terms. As a matter of fact, the insulating sheath thickness may be reduced when pressure increases, but on the other hand, the outer casing should then be thicker, and is therefore more expensive. As a rule, it is advantageous to raise said pressure as the service voltage increases. The pressure range currently used is of from 5 to 15 kg./cm.

When the cable is operating, water is pressurized in duct 11 and annular space 8. It has been found, by Way of experiment, that under such conditions, the service voltage of the cable is substantially raised. Moreover, said cable shows no particular impairment after a long time in service, in spite of the presence of water.

These facts may be accounted for in the following manner.

The thermoplastic material of sheath 5 is more or less pervious to steam. Therefore, there occurs at the surface of said sheath a vaporization of the water which, while being very limited, is still sufficient to cause diffusion of the steam into the plastic material and notably into the gaseous vacuoles present in the latter due to unavoidable manufacturing defects. The ionization level in said vacuoles is thus raised as a result of steam pressure.

Secondly, the extruded synthetic insulators, when of good quality, retain their inherent dielectric properties even when immersed in water.

Finally, the presence of water under pressure is attended by several other technical effects, viz:

By engaging the sheath 5, the water improves the continuity between the latter and the conductors 3. In particular, the gaseous vacuoles at the surface are eliminated.

The volume of water contained in the cable increases the total thermal capacity of the latter as compared with a cable of same cross-sectional area without water, thereby improving its resistance to overloads and short-circuits. Moreover, in certain cases, the water may act as an effective cooler for the cable, as will be explained subsequently.

Lastly, the electric conductivity of water, however low, plays a significant part by ensuring an electric connection between the cable conductors 3 and the insulating sheath 5, which is thus brought to the same potential a every point of its surface.

The various above-recited particular features, which have been ascertained by way of experiment, impart to the pressurizing of cables with solid thermoplastic insulating sheaths by means of water or other low conductivity liquid the character of a specific application.

The technical improvement afforded by the invention may be estimated as follows, in terms of reduction in the thickness of the insulating sheath as compared with cables with a solid thermoplastic sheath operating under normal pressure: 7

It will thus be appreciated that the insulator thicknesses may be reduced by at least 30%.

FIG. 3 shows a three-pole cable arranged according to the invention. Each conductor 3a, 3b, 3c is coated with an insulating sheath 5a, 5b, 5c of thermoplastic material provided with a conducting coating 6a, 6b, 6c.

The assembly is housed in a casing 7 adapted to withstand the aggregate pressure applied to the three conductors. Said casing may advantageously be made of steel, preferably lined with a thermoplasticvarnish or with a glazed enamel to prevent possible water pollution. According to the invention, each of said conductors is formed with an axial duct 11a, 11b, 11c filled with water and the space 8 between the casing 7 and the three conductors is also filled with water, the Whole being pressurized. When the casing 7 is rigid and is set in place before the conductors, in order to allow the conductors to be pulled easily into position within said casing, provision is advantageously made on the shield 6 of slide strips 12a,

12b, 12c of half circular configuration wound in highpitched helices. The strips12 are made of a non-magnetic material (metal or hard synthetic material).

Various end-boxes may be provided for the cables according to the invention. Said boxes are so arranged that the water in the central conductors, which is at the service voltage, is fed to the pressurizing devices,which are at the ground potential. Moreover, said boxes must be sodesigned that the ohmic losses in the'water do not result in an excessive heating ofthe assembly (this heat:

ing must be of the order of that occurring in the cable,

under full load conditions).

In the embodiment shown in FIG. 4, corresponding for example to a 15 kv.-cable, the casing 7 is connected to a coupler 17 which is clamped between packings 18 ar ranged on a bearing plate 19. Applied on the packings 18 is an insulating sleeve 22, for example of porcelain, secured to plate 19 by bolts, not shown.

' The sleeve 22 forms a central conduit 23 accommodating the end of the conductor coated with its sheath 5. The shield 6 terminates before the inlet of sleeve 22 and is tightened at its end in a ring 24 electrically connected to the coupler 17, said shield 6 being thus at the same po-v tential as casing 7, which is assumed to be made of metal.

Beyond sheath 5, the conductors 3 are inserted in a connecting terminal 25 formed with an axial passage 26 aligned with duct 11. Passage 26 leads to a transverse passage 27 opening onto a chamber 28 defined by a bellshaped member 29, secured to the end of sleeve 22.

A gasket 31 is provided to ensure the fluid-tight mounting of a rod 32, secured to theconnecting terminal 25, through the wall of chamber 28. Rod 32 acts as a sup port for a collar 33 wherebythe cable may be electrically connected to a conductor, not shown.

The duct 11 in the cable-thus communicates-with the annular space 8 through passages 26, 27, chamber 28 and the annular space 34 provided between the passage 23 in the sleeve and the sheath 5. This aggregate "volume is occupied by the water pressurized by a pump 35 which is connected to the coupler 17 through a pipe 36 and is fed with distilled water from a reservoir 37.

In this embodiment, the cross-sectional area of the Water annulus 34 'must bereducedto the lowest possible value so as to minimize the ohmic losses liable to occur in this area as a result of the slight electric conductivity always present in Water, however pure.

Practically, the limit to the permissible ohmic losses is 6 set by the extent of heating of the sleeve consistent with a satisfactory operation.

Of course, a safety valve may be provided in the upper portion of the bell-shaped member 29, in case untimely gas releases would take place.

In FIG. 4, the dimensions of the water anulus 34 have been exaggerated. In practice, this water annulus has a thickness ranging from one to a few millimeters for an end-box for about 15 kv. (i.e. a conductor/ ground voltage of 8.65 kv.). Should the internal diameter of sleeve 22 be too great, it could be reduced by insertion of a polyethylene tube of suitable dimensions. In the example illustrated above, with an end-box 30 cm. high, a cable having a sheath diameter of 50 mm. and an annular space 34 of a 2 mm. thickness, the losses for 0.5 megohms-cm. /cm. Water were found to be about 45 Watts.

The form of construction of FIGURE 5 shows the adaptation according to the invention of a cable and of its end-boxes to the case of a high power connection where artificial cooling has to be provided for permitting very high current densities.

In this embodiment, the two-end boxes 41a, 41b have a structure identical to that of FIG. 4, except for the difference that box 41a is provided at the level of coupler 17 with a partition 42 surrounding sheath 5, a fluidtight packing 43 being interposed therebetween. Partition 42 thus separates the annular space 8 from the water annulus 34.

The pressurizing pump 35, similar to that of FIG. 4, is on the other hand complemented by a circulation pump 44 fed through a pipe 45 which is connected to the coupler 17 through a purifying unit 46 and a cooling unit 47.

The pump 44 delivers into a pipe 49 which is connected to the coupler 17, but on the opposite side from pipe 45 with respect to partition 42.

The operation will be readily understood: while pump 35 maintains the water in the cable under pres sure, pump 44 serves to suck the water in sleeve 22 of box 41a which water comes from duct 11, and to deliver it into annular space 8. A circulation is thus created in the whole cable along the path shown by the arrows. g The output of pump 44 depends upon the ohmic losses in conductors 3 of the cable.

It will be understood that the single-pole system of FIG. 5 maybe applied to the case of a multiphase cable, the space 8 of FIG. 3 serving to return the water circulating in the three conductors. I

The following numeral example afiords some details on such an arrangement:

A three-phase connection for 2,000 amperes under a 30 kv. inter-phase service voltage (with neutral earthing) may consist of three single-pole cables including:

A copper conductor having a 790 mm. cross-sectional area with a 25 mm. central aperture.

A 5.7 mm. polyethylene insulating coating.

An outer shield made of copper.

Slide strips made of polyamide.

The three conductor assembly is pulled into a polyvinyl chloride pipe; purified water, under an average pressure of about 5 kg./cm. is circulated in the central cable aperture and in the vinyl chloride pipe to effect cooling. Assuming that the water is fed at 35 C. and that the maximum temperature isto be 70 C., the required rate of flow of water is 785 litres per hour and per conductor, i.e. a total flow of 2,255 litres per our.

In FIG. 6, there is shown a specific construction of an end-box comprising an internal coiled pipe for returning the water. More precisely, said end-box includes a sleeve 51, e.g. of porcelain, which acts as an insulator and is secured on a plate 52 by a threaded collar 53, in turn secured to the plate 19 by'bolts 54. The insulating sheath 5 of the cable beyond the shield 6 traverses the plate 52 through a packing 55 which prevents any water leakage into the space 56 within sleeve 51. The space 56 is filled with an insulating compound. 7/

(tar or plasticised polyethylene).

Sleeve 51 receives in its upper portion a removable plate 57 wherein the cable sheath extends through a packing 58. Said sheath terminates into a connecting terminal 59 with an extension in the form of a coupler 61 which has mounted thereon a pipe 62 bent into a crook-like configuration, which may for example be a copper pipe. A union-joint 63 connects pipe 62 with a coiled tube 64, made of polyvinyl chloride, which extends through the lower plate 52 through a packing 65. A second union-joint 66 connects the lower end 67 of the coiled tube 64 to a copper coupler 63, fitted 0n the outer insulating casing 7 of the cable.

The coiled tube thus serves to return the water from duct 11 to the annular space between casing 7 and shield 6.

, Moreover, the coiled tube permits of increasing the length of the Water annulus which acts as a leak line, whereby a suitable resistance may be provided even with water having a specific resistance of only 0.1 megohms cm. cm. 1

Thus, with a 15 kv. end-box equipped with a tube having a mm. inner diameter and a l-meter length, the losses may in this case be as low as watts.

What I claim is:

1. A high voltage electric cable under fluid pressure comprising within a fluid tight casing at least one core conductor and a solid insulating sheath of thermoplastic material surrounding said core, said material being selected from the group consisting of polyethylenes, polypropylenes, polyvinylchlorides, and copolymers thereof, with the inner surface of said sheath facing said core and the outer surface thereof facing said casing, an annular space located between said outer surface and said casing, an axial duct located within said core, pure water under a pressure substantially comprised between 1 and 25 kg./cm. housed within said annular space and said axial duct, said water having an electrical resistivity substantially ranging from 0.1 to 2 megohms-cm. /cm., and means affording a contact for said water with said inner and outer surfaces of said insulating sheath.

2. A high voltage electric cable according to claim 1' wherein said meansaifording a contact between said water and said insulating sheath comprisesa perforated tube housed within said core.

3. An end-box for a high voltage cable terminal, said cable comprising a fluid tight casing, at least one core conductor having an axial duct therein and a solid in sulating sheath of thermoplastic material thereabout housed within said casing, with the inner and outer faces of said sheath respectively facingsaid core and said casing, an annular space located between the outer face of said sheath and said casing, pure water under a pressure substantially comprised between 1 and 25 kg./cm. housed within said annular space and said axial duct, said water having an electrical resistivity substantially comprised between 0.1 and 2 megohms-cm. /cm., means affording a contact for said water with said inner arid outer surfaces of said insulating sheath, said end-box comprising an insulating sleeve, means for tightly fitting said sleeve on the casing terminal of said cable, with the terminal of said core and said insulating sheath accommodated within said sleeve, the inner diameter of said sleeve being larger than the outer diameter of said core terminal and of said insulating sheath, thereby forming an annular chamber therebetween, means providing a direct communication for said water between said annular chamber and said annular space within said casing, means providing further communication for said water between said axial duct and said annular chamber, and means comprising a pump in communication with said annular chamber for maintaining said water under pressure.

4. An end-box for a high voltage cable terminal,-

said cable comprising a fluid tight casing, at least one core conductor having an axial duct therein and a solid insulating sheath of thermoplastic material thereabout housed within said casing, with the inner and outer faces of said sheath respectively facing said core and said casing, an annular space located between the outer face of said sheath and said casing, pure water under a pressure substantially comprised between 1 and 25 kg./cm. housed within said annular space and said axial duct, said water having an electrical resistivity substantially comprised between 0.1 and 2 megohms-cm. /cm., means affording contact of said water with said inner and outer surfaces of said insulating sheath, said end-box comprising an insulating sleeve, means for tightly fitting said sleeve on the casing terminal of said cable, with the terminal of said core and said insulating sheath accommodated within said sleeve, the inner diameter of said sleeve being larger than the outer diameter of said core terminal and of said insulating sheath, thereby forming an annular chamber therebetween, said end-box further comprising a coiled tube .of an insulating material connected at one end to the terminal of said axial duct of said core conductor and connected at its other end with said annular space between said insulating sheath and said casing; said tube being also filled with water, at least a part of said coiled tube being located within said insulating sleeve, and an insulating compound housed within said sleeve around said coiled tube and said core terminal.

5. An installation for transmitting with an electric high voltage cable high intensity currents between two points, said installation comprising a cable comprising a fluid tight casing, and within said casing at least one core conductor having an axial duct therein and a solid insulating sheath of thermoplastic material around said core with the inner and outer faces of said sheath respectively facing said core and said casing, an annular space between said outer face and said casing, pure water under a pressure substantially comprised between 1 and 25 kg./cm. housed within said annular space and axial duct, said water having an electrical resistivity substantially. ranging from 0.1 to 2 megohms-cm. /cm., means affording contact of said water with said inner and outer surfaces of said insulating sheath, an endbox located at each of said points at both terminals of said cable, each of said end-boxes comprising an insulating sleeve, means for tightly fitting said sleeve to said casing of said cable, the terminal of said core and said insulating sheath being accommodated within said sleeve, with the inner diameter of said sleeve being larger than the outer diameter of said core terminal and of said insulating sheath, thereby forming an annular chamber therebetween, means providing a direct communication within each of said end-boxes between said inner duct at said core terminal and said annular chamber, said installation further comprising a cooling unit, and circulating means for said water through said axial duct, the annular space in said cable, the annular chambers in said sleeves of said end-boxes, and said cooling unit.

References Cited in the file of this patent Great Britain Apr. 13, 1960 

1. A HIGH VOLTAGE ELECTRIC CABLE UNDER FLUID PRESSURE COMPRISING WITHIN A FLUID TIGHT CASING AT LEAST ONE CORE CONDUCTOR AND A SOLID INSULATING SHEATH OF THERMOPLASTIC MATERIAL SURROUNDING SAID CORE, SAID MATERIAL BEING SELECTED FROM THE GROUP CONSISTING OF POLYETHYLENES, POLYPROPYLENES, POLYVINYLCHLORIDES, AND COPOLYMERS THEREOF, WITH THE INNER SURFACE THEEOF FACING SAID CASING, AN ANNULAR SPACE LOCATED BETWEEN SAID OUTER SSURFACE AND SAID CASING, AN AXIAL DUCT LOCATED WITHIN SAID CORE, PURE WATER UNDER A PRESSURE SUBSTANTIALLY COMPRISED BETWEEN 1 AND 25 KG./CM.2 HOUSED WITHIN SAID ANNULAR SPACE AND SAID AXIAL DUCT, SAID WATER HAVING AN ELECTRICAL RESISTIVITY SUBSTANTIALLY RANGING FROM 0.1 TO 2 MEGOHMS CM.2/CM., AND MEANS AFFORDING A CONTACT FOR SAID WATER WITH SAID INNER AND OUTER SURFACES OF SAID INSULATING SHEATH. 